My old boss used to do this as a hobby. He would go around to old houses and detect around the old outhouses. Sometimes he had to dig deep, but found interesting items no one went after they fell in.
Funny thing, growing up, the next door neighbor kid had a metal detector. He and I had many adventures. One with the detector, we got a real strong signal, right in the middle of my dad's strawberry patch. We adjusted it to supposedly discriminate somewhat, but probably did it incorrectly. Anyway, his dad and my dad came and looked. Had dad's permission to carefully remove the plants, and as long as we returned it to original, go wild. Well, big 8 x 8 tarp came out and a big square of plants removed. Still getting strong signal. kept digging, still getting strong signal (note, we did know where all the lines came in through our yard, that is important thing to do.) Eventually we caught the interest of the neighborhood. Soon, there were 10 men drinking beer cheering us on. Luckily, i lived in a river town and we had topsoil for days. Soon the hole was more than 5 feet deep, dreams of confederate gold, etc danced in our head even though it was Illinois. Then we hit it. There were 10 heads leaned over the hole as we cleared away the dirt from....... a coal bucket! We looked up to see only one head, my dad. "Get your a$$ out of that hole and fix my strawberry patch!" Well, that was the last adventure with the metal detector.